In the late 1950's, it was determined that internal combustion engine exhaust emissions were a significant contributor to the photochemical smoke and smog that enshrouded industrialized cities around the world. The damage to human health, animal life and the environment in general due to the effects of exhaust emissions have been widely studied and thoroughly documented. In order to reduce smog levels, governments have enacted legislation to control pollution emissions at their sources, including exhaust emissions from internal combustion engines.
Starting in the early 1960's, manufacturers of spark ignition (otto cycle) gasoline fuelled engines began installing the first rudimentary pollution abatement devices, some of which are still in use today. Over the years as automobile use has proliferated, government legislation has become more stringent with regard to otto cycle engine exhaust emission levels and the vehicle manufacturers have responded with more complicated equipment on their engines to comply with more stringent regulations. While the exhaust emissions of otto cycle engines have been strictly regulated, governments have largely ignored compression ignition engines (diesel engines) because those engines are more fuel efficient and have relatively lower emission levels than otto cycle engines without pollution control equipment. Another contributing factor was probably the much smaller number of diesel engines compared to the number of otto cycle engines in concurrent use.
As more was learned through research about the effects of exhaust emission caused pollution, governments legislated stricter controls of a broader range of such emissions. In the early 1980's, even diesel powered vehicle emissions began to be scrutinized and diesel engine manufacturers were forced to follow the suit of otto cycle powered vehicle manufacturers who applied a variety of increasingly complex strategies and apparatus in multi-pronged responses to government legislation regulating acceptable exhaust emission levels.
In the mid 1970's, otto cycle engine manufacturers developed a strategy for recycling a portion of the exhaust gases back into the intake manifold to subject that portion of recirculated exhaust to combustion conditions in order to control nitrogen oxides (NO.sub.x), carbon monoxide (CO), and total hydrocarbon emissions (THC).
Exhaust gas recirculation (EGR), by the introduction of gases from the exhaust into the combustion cycle, causes lower combustion chamber temperatures, thus inhibiting the formation of NO.sub.x as well as promoting the oxidation of some of the previously unburned hydrocarbons. The control of the recirculation of exhaust gases is performed by EGR valves which are widely used on otto cycle engines and, to a much less extent, on diesel engines.
Until the early 1990's, EGR valves were pneumatically actuated and controlled and thus were not capable of precise monitoring or rapid response to varying engine speeds and loads. The pneumatic actuation and control method also induces inaccuracies in valve positioning and delays in response time due to changing barometric pressures in the surrounding atmosphere. In the early 1990's, EGR valves controlled by microprocessor based engine controllers using electric actuator motors were introduced for otto cycle engines.
A system for controlling the amount of recirculated exhaust gas for a diesel engine is also known and described by Ikeda in U.S. Pat. No. 4,562,821 which issued Jan. 7, 1986. In this system, an electronic controller senses engine speed, intake manifold pressure, fuelling rate, engine coolant temperature and combustion flame brightness in order to control exhaust gas recirculation in a diesel engine. The system has two principal shortcomings. First, an expensive combustion flame brightness detecting system is required by the strategy. The sensors are not readily retrofit to existing engines because a special adaptation of the engine is required. Furthermore, the system relies on a vacuum actuated EGR valve which as noted above is slow to respond to rapidly changing engine operating conditions.
Another approach to the reduction of diesel engine emissions has been the development of dual fuel and multifuel systems for replacing a portion of the diesel fuel normally burned in compression ignition engines with a lighter, cleaner burning gaseous fuel such as natural gas. Extensive research has shown that EGR's compatibility with dual/multifuel engines to be quite different from otto cycle engines. In dual/multifuel engines, optimal EGR can vary between 0% and more than 50%, and manifold pressure differentials are quite low. It has therefore become apparent that an otto cycle engine EGR system and strategy are unsuitable for use in compression ignition engines in general, and in dual/multifuel compression ignition engines in particular.